Mr Fitzsimmons said Wednesday's weather conditions would be worse than initially feared.

"They are going to be worse to the point that the fire danger rating across most of the Greater Sydney area, the Hunter and indeed the fireground areas of the Blue Mountains and Southern Highlands, are going to be widespread extreme fire danger ratings."

Mr Fitzsimmons said temperatures would be in the mid to high 30s with wind gusts of up to 100 kilometres predicted.

"You've got a whole new ball game. It's more than just a bad weather day," he said.

Mr Fitzsimmons advised Blue Mountains residents to leave the area before lunchtime on Wednesday.

Thousands of firefighters and 90 aircraft are battling 57 fires, 17 of them uncontained, on the sixth day of the state's bushfire emergency.

The RFS announced all Blue Mountains schools, pre-schools and childcare centres would be closed, with some nursing homes evacuated.

Nine other schools will also be closed from Bargo in the southern highlands to Kurrajong in Hawkesbury.

Premier Barry O'Farrell says homes could come under threat again.

"We expect weather conditions to worsen tomorrow, meaning that fire activity is likely to increase and we could see homes under threat again," he told parliament on Tuesday.

More than 200 homes have already been destroyed by the fires.

Mr O'Farrell says the state is undergoing one of its largest ever mobilisations of fire fighters and fire utilities in preparation.

Emergency Services Minister Michael Gallacher is telling parents to keep their children close.

"It's not a day off where mum and dad can let the kids out tomorrow into the community," he told parliament.

"We are asking parents or those who are guardians of children to keep those children under your care, under your direction tomorrow because the situation is so fluid."

Communities expected to be most at risk include those along the Bells of Line Road, east of the Winmalee and Springwood area, and the Yarramundi Valley.

There will be an extra 1400 fire fighters on standby on Wednesday, but Mr Fitzsimmons says there is no guarantee of "a fire truck for every home and a message for every person".

The major blazes at Lithgow, Springwood and Mount Victoria are at "Watch and Act" level, with the same advice issued for a blaze at Balmoral, in the southern highlands.

The temperatures are expected to drop on Thursday but a dry change will bring strong southwesterly winds of up to 80 km/h through the fire-affected areas.

State forests in Sydney, the Hunter, the central west and the Southern Highlands will be closed on Wednesday because of the high fire danger.

Mr Fitzsimmons said the northeast corner of the state is also at risk.

Blue Mountains National Park, Kanangra-Boyd National Park, Wollemi National Park south of the Capertee River, all Hunter and Central Coast national parks and all fire-affected Port Stephens parks will also be closed to the public.